Optical Interconnects

The subject application claims the benefit of priority to U.S. Provisional Patent Application No. 63/683,253 filed on Aug. 15, 2024, titled “Optical Interconnects,” which is incorporated by reference herein in its entirety for all purposes.

This application relates to implementations of optical interconnects for high-speed communications.

High-performance computing applications such as neural network model trainings or inferences for artificial intelligence (AI) applications require high bandwidth communications among various computing elements and/or memory elements. As an example, certain computing elements such as graphics processing units (GPU) require intense parallelism, and moving massive data off and between chips can be the bottleneck of the computing performance and energy efficiency. Optical interconnects (or optical links) offer significant advantages in such high-performance computing applications. One of the primary benefits is the high bandwidth that optical interconnects provide, enabling much faster data transfer rates compared to electrical interconnects. This is critical in neural networks, where vast amounts of data must be processed and transmitted between nodes rapidly. Additionally, optical communication can significantly reduce latency, which is vital for the performance of real-time processing tasks in robotics or edge computing. Another major advantage of optical interconnects is energy efficiency. They consume less power than electrical interconnects, particularly over long distances. Lastly, optical signals are immune to electromagnetic interference and crosstalk, leading to reduced signal degradation and more reliable data transmission for maintaining the accuracy of high-performance computations.

Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments.

One example aspect of the present disclosure is directed to an optical interconnect that includes a transmitter unit and a receiver unit. The transmitter unit includes a laser device configured to generate a source optical signal, a modulator unit configured to modulate the source optical signal and to generate a modulated optical signal having an in-phase component and a quadrature component, and a first optical coupler configured to couple a transmitted optical signal having the modulated optical signal out of the transmitter unit. The source optical signal and the modulated optical signal propagate in-plane along a surface of the transmitter unit, and the transmitted optical signal propagates out-of-plane from the surface of the transmitter unit. The receiver unit includes a second optical coupler configured to couple the transmitted optical signal into the receiver unit, and a demodulator unit configured to receive the transmitted optical signal and a reference optical signal and generate a demodulated electrical signal having the in-phase component and the quadrature component.

In some implementations, the transmitter unit may further include a first digital signal processor configured to receive one or more input data signals and to generate one or more DSP signals representing the in-phase component and the quadrature component; and one or more first digital-to-analog converters configured to receive the one or more DSP signals from the first digital signal processor and to generate one or more analog control signals for driving the modulator unit.

In some implementations, the receiver unit may further include one or more first analog-to-digital converters configured to receive the demodulated electrical signals and to generate one or more demodulated digital signals; and a second digital signal processor configured to receive the one or more demodulated digital signals and to generate one or more output data signals representing the one or more input data signals.

In some implementations, the modulator unit may include nested Mach-Zehnder interferometers and a quadrature phase-shifter. In some other implementations, the modulator unit may include nested ring modulators and a quadrature phase-shifter.

In some implementations, the first optical coupler and the second optical coupler may include optical grating couplers, where the transmitted optical signal propagates out-of-plane from the surface of the transmitter unit at a substantially normal direction from the surface of the transmitter unit.

In some implementations, the transmitter unit may further include a second modulator unit configured to modulate the source optical signal and to generate a second modulated optical signal having a second in-phase component and a second quadrature component, where the transmitted optical signal further includes the second modulated optical signal. The receiver unit may further include a second demodulator unit configured to receive the transmitted optical signal and the reference optical signal and generate a second demodulated electrical signal having the second in-phase component and the second quadrature component.

In some implementations, the first optical coupler and the second optical coupler may include latticed grating couplers having two inputs.

In some implementations, the reference optical signal may be generated by the receiver unit. In some other implementations, the reference optical signal may be generated by the transmitter unit.

In some implementations, the modulated optical signal may represent a phase-shift-keying (PSK) modulated signal. In some other implementations, the modulated optical signal may represent a quadrature amplitude modulation (QAM) modulated signal.

In some implementations, the optical interconnect may include an optical cable that is optically coupled to the first optical coupler of the transmitter unit and the second optical coupler of the receiver unit. The optical cable may include one or more single-mode fibers or one or more polarization maintaining fibers. In some implementations, one end of the optical cable may be co-packaged with the transmitter unit via a first fiber array having multiple fibers, and another end of the optical cable may be co-packaged with the receiver unit via a second fiber array having multiple fibers.

In some implementations, the modulator unit and the first optical coupler may be integrated in a silicon substrate. In some implementations, the laser device may be bonded to the silicon substrate.

Another example aspect of the present disclosure is directed to a computing system that includes a first processor element, a second processor element, and an optical interconnect configured to provide data communications between the first processor element and the second processor element. The optical interconnect includes a transmitter unit and a receiver unit. The transmitter unit includes a laser device configured to generate a source optical signal, a modulator unit configured to modulate the source optical signal and to generate a modulated optical signal having an in-phase component and a quadrature component, and a first optical coupler configured to couple a transmitted optical signal having the modulated optical signal out of the transmitter unit. The source optical signal and the modulated optical signal propagate in-plane along a surface of the transmitter unit, and the transmitted optical signal propagates out-of-plane from the surface of the transmitter unit. The receiver unit includes a second optical coupler configured to couple the transmitted optical signal into the receiver unit, and a demodulator unit configured to receive the transmitted optical signal and a reference optical signal and generate a demodulated electrical signal having the in-phase component and the quadrature component.

In some implementations, the first processor element, the second processor element, and the optical interconnect may be co-packaged on a common substrate. In some implementations, each of the first processor element and the second processor element may include one or more of a graphics processing unit (GPU), a central processing unit (CPU), a neural network processing unit (NPU), or a tensor processing unit (TPU).

Other example aspects of the present disclosure are directed to systems, methods, apparatuses, sensors, computing devices, tangible non-transitory computer-readable media, and memory devices related to the described technology.

These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure, and together with the description, serve to explain the related principles.

Like reference numbers and designations in the various drawings indicate like elements.

The present disclosure describes optical interconnects implemented based on photonics integrated circuit. In general, pluggable optical interconnects that are edge-coupled to an external medium (e.g., an optical fiber or an optical fiber array) may have limited achievable data rate per interface, higher packaging costs (e.g., complex optical alignment for edge-coupled devices), and higher testing cost (e.g., wafer-level testing is not possible for edge-coupled devices). For highly parallel computing architectures, there are technical advantages for implementing optical interconnects that can be vertically coupled off-chip to an external medium, so to achieve high performance, better area efficiency, and low cost. Moreover, optical interconnects implemented using a coherent scheme based on photonics integrated circuit further increases overall transmission bandwidth per channel. A coherent scheme uses both the amplitude and phase (and in some cases, polarization) of light to encode information, which enables higher data rates, longer transmission distances, and better spectral efficiency. Lastly, in addition to pluggable optical interconnects, optical interconnects can be co-packaged to be closer to the computing elements, in order to improve the bandwidth density and energy efficiency for data link without incurring substantial energy cost.

1 FIG.A 100 100 110 170 120 100 a a a shows an example coherent optical interconnect. The coherent optical interconnectincludes a transmitter unitand, an optical cable, and a receiver unit, where the coherent optical interconnecttransports data between two or more computing/memory elements. Each of the computing/memory elements can include one or more of a graphics processing unit (GPU), a central processing unit (CPU), a neural network processing unit (NPU), a tensor processing unit (TPU), a memory (e.g., high bandwidth memory, random access memory, etc.), or a combination thereof. The transported data may be modulated using a coherent modulation scheme such as a phase-shift-keying (PSK) modulation or a quadrature amplitude modulation (QAM) modulated signal, depending on the required effective data rate.

110 130 110 140 130 114 130 140 The transmitter unitincludes a first digital signal processor (DSP)configured to receive one or more input data signals from a computing/memory element, and to generate one or more DSP signals representing the in-phase component (the “I” component) and the quadrature component (the “Q” component) of encoded symbols. The transmitter unitfurther includes one or more first digital-to-analog converters (DAC)configured to receive the one or more DSP signals from the first DSPand to generate one or more analog control signals for driving an optical I-Q modulator(or a modulator unit, used interchangeably with the term “optical I-Q modulator”). The DSPand the DACmay be implemented on different chips or on the same chip.

110 112 112 112 112 112 The transmitter unitfurther includes a laser deviceconfigured to generate a source optical signal. In some implementations, the laser devicemay be modulated at a reference frequency and amplitude. In some implementations, the laser devicemay be integrated with a photonic integrated circuit. For example, the laser devicemay be bonded to a silicon substrate (e.g., III-V lasers hybrid-bonded to a silicon waveguide formed in a silicon-on-insulator (SOI) substrate). In some other implementations, the laser devicemay be an external laser source that couples optical signals to the photonic integrated circuit via an external medium (e.g., an optical fiber).

110 114 114 114 140 114 The transmitter unitfurther includes an optical I-Q modulatorconfigured to modulate the source optical signal and to generate a modulated optical signal having the in-phase component and quadrature component. In some implementations, the optical I-Q modulatormay be implemented using nested Mach-Zehnder interferometers (MZI) and a quadrature phase-shifter arranged to perform I-Q modulations. In some other implementations, the optical I-Q modulatormay be implemented using nested ring modulators and a quadrature phase-shifter arranged to perform I-Q modulations. The one or more analog control signals generated by the first DACare used to drive the optical I-Q modulator(e.g., control the phase changes along different MZI arms, or rings, etc.).

110 116 110 114 116 110 116 110 The transmitter unitfurther includes a first optical couplerconfigured to couple a transmitted optical signal having the modulated optical signal out of the transmitter unit. In some implementations, the optical I-Q modulatorand the first optical couplermay be integrated in a silicon substrate (e.g., silicon waveguides and grating couplers formed in a silicon-on-insulator (SOI) substrate) or a suitable semiconductor or III-V-materials substrate, where the source optical signal and the modulated optical signal propagate in-plane along a surface of the transmitter unit. In some implementations, the first optical couplermay be formed as an optical grating coupler, where the transmitted optical signal propagates out-of-plane (e.g., substantially normal from the chip surface) from the surface of the transmitter unitto enable easier packaging and optical alignment.

120 126 120 120 124 124 126 124 The receiver unitincludes a second optical couplerconfigured to couple the transmitted optical signal into the receiver unit. The receiver unitfurther includes an optical I-Q demodulator(or a demodulator unit, used interchangeably with the term “optical I-Q demodulator”) configured to receive the transmitted optical signal and a reference optical signal, and to generate a demodulated electrical signal comprising the in-phase component and/or the quadrature component. In some implementations, the optical I-Q demodulatormay be implemented using nested 3-dB couplers, a quadrature phase-shifter, and photo-detectors. The second optical couplerand the optical I-Q demodulatormay be integrated in a silicon substrate or a suitable semiconductor or III-V-materials substrate.

120 122 112 110 120 110 112 120 118 128 1 FIG.A 1 FIG.B In some implementations, the reference optical signal may be generated by the receiver unit. Referring toas an example, the reference optical signal may be generated by a local referencethat is synchronized in phase with the laser device. In some other implementations, if the distance between the transmitter unitand the receiver unitis short (e.g., chip-to-chip or blade-to-blade transmission), the reference optical signal may be generated by the transmitter unit. Referring toas an example, the reference optical signal may be generated by the laser deviceand transmitted to the receiver unitvia optical couplersand.

120 150 120 160 160 150 The receiver unitfurther includes one or more first analog-to-digital converters (ADC)configured to receive the demodulated electrical signals and to generate one or more demodulated digital signals. The receiver unitfurther includes a second digital signal processor (DSP)configured to receive the one or more demodulated digital signals and to generate one or more output data signals representing the one or more input data signals to the second computing/memory element. The DSPand the ADCmay be implemented on different chips or on the same chip.

170 110 120 170 110 170 120 170 170 The optical cableis optically coupled to the transmitter unitand the receiver unit, where one end of the optical cablemay be co-packaged with the transmitter unitvia a first fiber array having one or multiple fibers, and the other end of the optical cablemay be co-packaged with the receiver unitvia a second fiber array having one or multiple fibers. In some implementations, the optical cableincludes one or more single-mode fibers. In some other implementations, the optical cableincludes one or more polarization maintaining fibers.

116 118 126 128 116 118 126 128 In some implementations, each of the optical couplers///may be a 1-D grating coupler having one input. In some other implementations, the optical couplers///may be latticed grating couplers having two inputs. A latticed grating coupler is a 2-D grating coupler formed by superimposing two 1-D grating couplers oriented at orthogonal directions (e.g., a first 1-D grating along the x-direction, and a second 1-D grating along the y-direction). Such latticed grating couplers allow two optical signals to be transmitted in parallel in a single optical fiber having two orthogonal propagation axes (e.g., a fast axis and a slow axis), and therefore doubling the data rate.

2 FIG.A 1 FIGS.A 210 110 1 214 114 116 214 116 170 210 170 170 Referring toas an example, the transmitter unitis similar to the transmitter unitas described in reference to/B, and further includes a second optical I-Q modulatorconfigured to modulate the source optical signal and to generate a second modulated optical signal including a second in-phase component (I′) and a second quadrature component (Q′). The first modulated optical signal from the first optical I-Q modulatoris coupled to a first input of the first optical coupler(a latticed grating coupler), and the second modulated optical signal from the second optical I-Q modulatoris coupled to a second input of the first optical couplerthat is orthogonal to the first input. The optical cableis aligned with the transmitter unitsuch that the first modulated optical signal propagates along one optical axis of a fiber in the optical cable, and the second modulated optical signal may propagate along an orthogonal optical axis of the same fiber in the optical cable.

220 120 1 224 126 170 124 224 124 224 150 160 1 FIGS.A The receiver unitis similar to the receiver unitas described in reference to/B, and further includes a second optical I-Q demodulator. Here, the second optical couplermay be a latticed grating coupler that receives the transmitted optical signal from the optical cable, and splits the first modulated optical signal and the second modulated optical signal to the first optical I-Q demodulatorand the second optical I-Q demodulator, respectively. The first optical I-Q demodulatoris configured to receive the first modulated optical signal and the reference optical signal, and to generate a first demodulated electrical signal having the first in-phase component (I) and/or the first quadrature component (Q). The second optical I-Q demodulatoris configured to receive the second modulated optical signal and the reference optical signal, and to generate a second demodulated electrical signal having the second in-phase component (I′) and/or the second quadrature component (Q′). One or more of the first in-phase component (I), the first quadrature component (Q), the second in-phase component (I′), and/or the second quadrature component (Q′) can then be processed by the one or more first ADCand the second digital signal processorto generate the output data for the second computing/memory element.

220 122 112 210 220 210 112 220 118 128 2 FIG.A 2 FIG.B In some implementations, the reference optical signal may be generated by the receiver unit. Referring toas an example, the reference optical signal may be generated by a local referencethat is synchronized in phase with the laser device. In some other implementations, if the distance between the transmitter unitand the receiver unitis short (e.g., chip-to-chip or blade-to-blade transmission), the reference optical signal may be generated by the transmitter unit. Referring toas an example, the reference optical signal may be generated by the laser deviceand transmitted to the receiver unitvia optical couplersand.

100 100 200 200 300 300 300 300 a b a b b c d a 3 3 FIGS.B andC 3 FIG.D 3 FIG.A In some implementations, the first computing/memory element, the second computing/memory element, and the coherent optical interconnect (e.g., the coherent optical interconnect///) are co-packaged on a common substrate (e.g., PCB substrate, silicon interposer, silicon substrate, or any suitable common substrate, such as the computing systemandas described in reference to). In some implementations, the transmitter unit of a coherent optical interconnect is co-packaged with a first computing/memory element, and the receiver unit of the coherent optical interconnect is co-packaged with a second computing/memory element (e.g., the computing systemas described in reference to). For example, the coherent optical interconnect may be co-packaged with two GPUs on the same board to facilitate data communications between the two GPUs. In some implementations, the coherent optical interconnect may be a pluggable active optical cable (AOC) (e.g., the computing systemas described in reference to).

3 FIG.A 300 300 310 330 350 360 380 300 330 350 330 360 350 360 310 a a a shows an example of a computing system. The computing systemincludes an optical link, a board, an interposer, a processor, and a connector. The computing systemmay be implemented in a high-performance computing or networking environment such as a data center infrastructure for parallel computing and/or artificial intelligence (AI) applications (e.g., computations for large language model trainings and/or inferences). In general, the boardis a circuit board such as a server blade. The interposeris packaged (e.g., bonded) on the board, and is a specialized substrate (e.g., a silicon interposer) used in semiconductor packaging (e.g., 2.5D or 3D IC packaging) to facilitate the connection and integration of multiple chips or dies within a single package. The processoris packaged (e.g., bonded) on the interposer, and can include one or more of a graphics processing unit (GPU) chip, a central processing unit (CPU) chip, and/or a neural processing unit (NPU) chip. The processormay access data from another computing element (e.g., another processor and/or memory element) externally via the optical link.

310 320 110 210 120 220 370 170 310 310 310 390 310 330 380 310 330 310 330 310 350 310 360 3 FIG.A 3 FIG.B 3 FIG.C 3 FIG.D In general, the optical linkincludes an optical transceiver(e.g., transmitter unit/and receiver unit/) and a fiber array unit(e.g., optical cable). The optical linkmay be used for chip-to-chip, module-to-module, package-to-package, board-to-board, or any other suitable type of data communications. In some implementations, if the optical linkis a pluggable optical interconnect, the optical linkmay be packaged as a module with a PCB. Referring to, in some implementations, the optical linkmay be coupled to the boardvia a connector(e.g., a QSFP (Quad Small Form-factor Pluggable) connector). in some implementations, the optical linkmay be packaged (e.g., bonded) on the board. Referring to, in some other implementations, the optical linkmay be packaged (e.g., bonded) on the board. Referring to, in some other implementations, the optical linkmay be packaged (e.g., bonded) on the interposer. Referring to, in some other implementations, the optical linkmay be packaged (e.g., bonded) on the processor.

Although only a unidirectional transmitter unit/receiver unit is described here, a coherent optical interconnect can be bidirectional, and/or multi-channel by duplicating the transmitter unit/the receiver unit/number of fibers in a fiber array as needed in a particular application.

As used herein, the terms such as “first”, “second”, “third”, “fourth” and “fifth” describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another. The terms such as “first”, “second”, “third”, “fourth” and “fifth” when used herein do not imply a sequence or order unless clearly indicated by the context. The terms “photo-detecting”, “photo-sensing”, “light-detecting”, “light-sensing” and any other similar terms can be used interchangeably.

Spatial descriptions, such as “above”, “over,”, “under”, “top”, and “bottom” and so forth, are indicated with respect to the orientation shown in the figures unless otherwise specified. It should be understood that the spatial descriptions used herein are for purposes of illustration only, and that practical implementations of the structures described herein can be spatially arranged in any orientation or manner, provided that the merits of embodiments of this disclosure are not deviated by such arrangement.

As used herein and not otherwise defined, the terms “substantially” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can encompass instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can encompass a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

While the concepts have been described by way of examples and in terms of embodiments, it is to be understood that the disclosure is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.